Transmission structures of this type and their mode of operation are known, for example, from German Patent Application 101 02 028 A1. In parallel shift transmissions of this type, the torque of the drive unit is generally transmitted to a transmission output shaft and, thus, via a differential to the drive wheels in such a manner that a gear ratio step is engaged in a partial transmission between the transmission input shaft and the transmission output shaft and the friction clutch associated with this transmission input shaft is connected to the drive shaft of the drive unit. On the other transmission input shaft, for the disengaged friction clutch that is associated with this transmission input shaft, a gear ratio step is generally already shifted with which the vehicle is to be operated further after a shift operation. In this context, a shift operation is carried out as an overlap shift in which the clutch transmitting the torque at the time is gradually disengaged while the other friction clutch is engaged. Moreover, it may be advantageous to operate the motor vehicle, for example, during a starting off operation using a power split when gear ratios are engaged in both partial transmissions, at least one of the two clutches being operated in a slipping state as a result of the varying differential speeds between drive shaft and transmission input shafts.
When there are gear ratios engaged on both transmission input shafts, situations that are unfavorable for driving safety may arise, for example, at an unsuitable speed of a transmission input shaft. Moreover, components of the drive train, such as the transmission, the clutches or the drive unit may be destroyed or their service life reduced.